Comparative toxicity responses of goats fed on Leucaena leucocephala in Australia and Hawaii

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Abstract
To examine the apparent absence of toxicity in ruminants fed on Leucaena leucocephala in Hawaii, U.S.A., goats were offered comparable diets of Leucaena and lucerne chaff (control) in both Australia and Hawaii over a 7-week period. Intake of mimosine, excretion of the ruminal metabolite 3-hydroxy- 4(1H)-pyridone (DHP), and serum concentrations of thyroxine (T4) were measured in both studies. Mean mimosine intakes on the highest Leucaena diets in Australia and Hawaii were similar at about 20 g day-1. In Australia, goats on the all-Leucaena diets became hypothyroid after only 3 weeks of feeding, thyroid glands were enlarged and erosion of the oesophageal mucosa and reticulo-rumen occurred. Excretion of DHP in the urine was related to the daily mirnosine intakes, with recoveries of about 86%. In marked contrast, goats fed on Leucaena in Hawaii exhibited no clinical signs of toxicity and excreted less than 1% of the mirnosine intake as DHP in the urine. No degradation of DHP occurred in vitro with rumen fluid from Australian goats, whereas 71% of the added DHP was degraded after 5 h with rumen fluid from goats in Hawaii. The results support the hypothesis that the differences observed are attributable to a different microbial metabolism of mimosine and DHP in ruminants in Hawaii.

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